Transformed, hybrid strains of the yeast Saccharomyces capable of simultaneous secretion of both glucoamylase and ${alpha}-amylase$ have been produced. These strains can carry out direct, one-step assimilation of starch with conversion efficiency greater than 93% during a 5 day growth period. One of the transformants converts 92.8% of available starch into reducing sugars in only 2 days. Glucoamylase secretion by these strains results from expression of one or more chromosomal STA genes derived from Saccharomyces diastaticus. The strains were transformed by a plasmid(pMS12) containing mouse salivary ${alpha}-amylase$ cDNA in an expression vector containing yeast alcohol dehydrogenase promoter and a segment of yeast $2{mu}$ plasmid. The major starch hydrolysis product produced by crude amylases found in culture broths is glucose, indicating that ${alpha}-amylase$ and glucoamylase act cooperatively.